Method of making an alkaline earth metal carbonate of improved color



Patented Mar. 12, 1935 1 METHOD or MAKING AN ALKALINE EARTH 3 METALCABBONATE OF'IMPROVED COLOR 1 Church/and Raymond B. McClure,

Painesville, Ohio, assignorsjto-Pure Calcium I ration of-Qhio Products Company, Painesville, Ohio, a corpo- No Da ing; Application F bruary 23, 1933', 1

Serial .No. 658,147 q r 6 Claims (or-134. 48)

-This invention :relates generallyto alkaline earth metal carbonatesandthe method of. making the same, and more particularly to-.alkaline earth metal carbonates having improved; color whitenessand to the .method ofmaking the caribonateofiimp'roved whiteness. v i

' By the term alkaline earth metal carbonates itis intended to include calcium carbonate, barium carbonate, strontium carbonate-and magnes-ium carbonate. Although the invention is applicable tor'any of these carbonates, it is described herein more particularly as applied to calcium carbonate either in its naturally occur- :ring form .as lim'estoneor as produced. bya'ny =manufacturing process.

:stone in'varying degrees of purity. It usually contains, in addition. to impurities such as :silica :and flint, .a certain'am'ount' of iron which generx-ally is present as the oxide; :Theiron gives a reddish, yellowish or brownishcast to the calcium carbonate.-. If the limestone containing iron is ground to a fineness suitable .forpigment or filler purposesftheresultantjpowder in the dry state is apparently lighteriainscolorl' than the solid stone, but if the powder is wetted, the color reverts back nearly to that of the limestone.

The color of limestone may be improved to a certain extent by the mechanical elimination of coarse impurities and the oxidation of organic impurities. This may be accomplished by calcining the limestone to produce calcium oxide, slaking the oxide, recarbonating to form calcium carbonate, and drying. By this method, however, the discoloration due to iron still persists, resulting usually in a yellow, brown or red cast in the finished product, the degree of discoloration depending upon the amount of iron present in the material.

Due to the fact that iron and calcium compounds possess properties which are very similar, it is difficult to remove the iron economically.

We have found, however, that the discoloring effect of the iron present in the limestone may be greatly minimized and in some cases eliminated by the following procedure in which it is not necessary toresort to the removal of the iron from the calcium carbonate. Instead of removing the iron, it is converted into a form in which it has a less discoloring effect on the calcium carbonate.

The limestone is calcined to convert it into calcium oxide and the oxide is then slaked in hot water. Solid impurities such as silica, flint, unburned limestone, etc., are eliminated by suitable torial value.

'mechanical means such as screening, settling or ;other types of separation. The calcium hydroxide is reduced to a water slurry containing from 5% to. 30% .CaO by Weight. This slurry is an alyzed ioriron and. an amount of asulphide ion 5 in suitable form is added totheslurry suincient to slightly more than satisfy the; iron indicated by the analysis. The sulphide ion maybe supplied in the form of any soluble salt such, for example, as sodium sulphide, potassium sulfide, am-

' monium sulfide, or as a gas such as hydrogen sulphide. The addition of the sulphide convertsthe iron into ferrous sulphide which is blue and possessesumuch less tinting power than ferric oxide .or hydroxide.

.Calciumcarbonate' occurs in nature as lime- It is probably true that due to thehigh concentration of calcium hydroxide at the timethe sulphide is introduced, there is a temporary equilibrium between iron and calcium sulphide .ii hydrogen sulphide be used, and between iron,

calcium and-for instance, sodium sulphides if sodium sulphide be used. In any event, there is little perceptible change in the slurry at themomentof introductionof .the sulphide. The mixture isthen reacted with a carbonating agent I such, for example, as carbon dioxide gas, sodium carbonate, sodium bicarbonate, ammonium carbonate, or any other reactablecarbonating agent. As carbonating progresses, the slurry takes on a bluish cast, the extent of the blue coloration being determined by the amount of ferrous sulphide present. The iron present in the slurry either as iron oxide or hydroxide is converted by this process into iron sulphide as the calcium carbonate particles are formed, and it is believed that the iron sulphide is occluded within the calcium carbonate particles as well as being exposed in suspension. The filtered and dried calcium carbonate thus produced possess a greatly improved whiteness and brightness.

It is believed that the improved whiteness and brightness ofthe calcium carbonate produced by our method is due to the following conditions and reactions occurring in the process, although we do not wish to be bound by our theory. Ferric oxide or hydroxide is brown, red or yellow, depending upon its concentration, and possesses high tinc- Ferrous sulphide (formed by the action of the sulphide ion, which is at the same time a reduction agent) is insoluble in water, is 5 blue, and possesses much less tinting power than ferric oxide or hydroxide. The iron oxide or hydroxide is insoluble in water and is present in the slurry, colloidally dispersed throughout the slurry, and occluded within the particles of calof insoluble calcium carbonate, more calcium hydroxide is dissolved fromnthe.lundissolvedlsuse pended portion and more ironis 1iberated,' These are, in turn, acted upon by, the carbonating agent and the sulphide, respectively, ;and; the jpro 'ce ss proceeds until calcium hydroxide is"c'on'vei"ted' into calcium carbonate, and the iron oxide or hy droxide are converted into ferrous sulphi'da' We have described the process particularly as applied to improving the color whitehess and brightness of calcium carbonate. It Will be unders'tood, however, that the invention isapplicable to the treatment of otherhalkaline earth metal carbonates as hereinbefore mentioned and that the invention is notlimited to the method.v which 'has been described in detail, but may be otherwise embodied or practiced within the scope of the following claims.

Weclaim: l H '1. The method 'of V producing alkaline earth metal carbonates of improved color, which comprises treating a slurry containing an alkaline earth'metal hydroxide and iron in the form of ironoxide or iron hydroxide with a suitable sulphide and .carbonating agent innamount sufiicient to convert the principal part of the alkaline earth metal hydroxideto carbonate and the iron oxide or hydroxideto ironsulphide.

- 2. The method of producing calcium carbonate -of-improved color which comprises treatinga 40 -the-form of iron oxide or iron hydroxide. with a suitable sulphide and carbonatingcagent in slurry containing calcium hydroxideand iron in amount sufiicientto convertlthe principalpart .of

the calcium-hydroxide to calcium carbonate and the iron oxide or hydroxide to iron sulphide As calcium hy;

calcium; '0

3. The method of improving the whiteness of alkaline earth metal carbonates containing iron compounds, whichcomprises calcining the can bonate, slaking it and forming a slurry of alkaline earth metal hydroxide, treating the slurry with a suitable "sulphide andfaffsuitable iarbbnating agent, in amount suflicient" to'conve'rt the'pi'incipal part of the alkaline earth metal hydroxide into "carbonate, and the iron oxide or, hydroxide presentjn theslurry into iron sulphide, removing rthenpre ipitat ii d d yi i fl'hemethod of improving the whiteness of v,

I x y g ohategcontaining iron compounds, whichfcom rises calcining the carbonate, slaking ririilzilg' a' s'lurry of calcium hydroxide,

H y with a suitable sulphide and a jlsuitable carbonatin'g agent in amount suiiicient to convert the principal part of the calcium hydroxidejnto calcium carbonate and the iron oxide or hydroxide present in the slurry into iron sulphide, removing the precipitate, and drying-it. 5. The method of improving the whiteness -ot alkalinezearth metal carbonates'containing iron compounds iw'hich comprises calcining; thelcara bonate, slaking it and forming a'slurryof alkaline earth metal hydroxide, treating'the islurry with hydrogen sulphide and. a suitable :i :carbonating agent in amount suificient .to convertthe-principal part of the alkaline earth metal -hydroxide into carbonateUand-vthe iron oxide or hydroxide present in the slurry into iron sulphide, removing the precipitataand drying it.

bonate; slaking it and forming aslurry, of'alkaline 6. The methodiof improvingthe whitenesswof alkaline earthmetal carbonates containing iron compounds, which comprises calcining'the' car-j earth metalhydroi'zide, treating .theslurrywith carbonate and the iro'noxideor hydroxide present precipitate, anddryingzit. s, s 7 s. P JOHN'W.CHURCH. .Q;:

. RAYMOND RMCCLUREA hydrogen sulphide and'then with carborrdioxide gas in amount suflicient to convertrthe principal part of thenalkaline earth metal-hydroxideriirito-QO .in:the slurry vinto ironsul'phide, removing t-he 

